The present invention relates to the selective chemical processing of front side, back side and perimeter edge surfaces of microelectronic substrates.
This invention pertains to treating a substrate such as a semiconductor wafer, e.g., a silicon wafer, so as to remove a thin film, such as a copper or other metal or oxide film, from selected regions on the wafer.
The fabrication of a microelectronic circuit and/or component from a substrate typically involves a substantial number of processes. Many of these processes involve the deposition of a thin film on the surface of the workpiece followed by contact with a processing liquid, vapor, or gas. In a known process for treating a microelectronic workpiece, such as a silicon wafer, on which microelectronic devices have been fabricated and which has a front, device side, a back, non-device side, and an outer perimeter, thin-film layers are successively applied and etched to form, for example, a metallized interconnect structure. In a typical metallization process, both sides of a semiconductor wafer are coated with a protective film such as a silicon nitride or a silicon oxide. Thereafter, a barrier layer such as titanium nitride, tantalum or tantalum nitride is applied over a dielectric layer on the front side of the workpiece. Depending upon the particular process used to form the interconnect structures, the dielectric layer may include a pattern of recessed micro-structures that define the various interconnect paths. A thin metal film, such as a copper film is applied exterior to the barrier layer. In most instances, the thin film serves as an initial seed layer for subsequent electroplating of a further metal layer, such as a further copper layer. Due to manufacturing constraints, the thin film is not applied over an outer, peripheral margin of the front side.
Known techniques, such as physical vapor deposition (sputtering) or chemical vapor deposition, are typically used to apply the barrier layer and the thin film. Both methods can deposit copper onto the wafer bevel (the peripheral edge of the wafer), and in many cases this deposit is non adherent and can flake off in subsequent processing steps such as annealing or CMP. After deposition of the barrier layer, additional layers may be deposited to the wafer front side edge. In instances in which a further metal layer is to be electroplated exterior to the thin film, one or more electrical contacts are connected to an outer margin of the thin film to provide plating power. Because subsequent layers are deposited with an edge exclusion, the previously deposited layers are left exposed. Many of these layers allow copper to be deposited on them, but the adhesion is very poor and flaking during post processing is observed. A typical copper example might be an exposed barrier layer such as Ti/TiN being exposed to copper plating solution. Following electrochemical deposition, the barrier layer would have a copper film of low quality which would flake off easily in CMP. Removal of flaking material before CMP processing is desirable as the flakes have the potential to cause scratches in the polished surface, resulting in yield losses.
The surface area of the front side beyond the inner boundary of the outer margin of the thin film is not available for fabricating the microelectronic devices since the present manufacturing processes limit the extent to which device structures can be formed at the outer margin. It would be highly desirable and would result in increased yield if more of the surface area beyond the present limits of the outer margin of the thin film were available for fabricating interconnect structures.
Covering the exposed barrier layer with a full coverage seed layer would eliminate copper metal from flaking off the barrier and also have the added benefit of increasing usable area on the wafer surface. Even in this case, copper deposited on the bevel during the seed layer and electrochemical deposition would need to be removed, as it too can flake off and/or cause cross contamination of metrology tools. A clear area inboard of the wafer bevel may also be necessary for reliable processing; many clamp rings are very sensitive to surface characteristics.
In the known process discussed above, and in other processes, contamination by copper, other metals, or other contaminants can occur on the back side of the workpiece. Although copper and other metals tend to diffuse rapidly through silicon or silicon dioxide, the back side is generally not provided with barrier layers that are capable of preventing copper, other metals, or other contaminants from diffusing through the silicon wafer to the front side, at which such contamination can be very detrimental to device performance.
Such contamination can result from overspraying or other processing artifacts or from cross-contamination via fabrication tools. Such contamination can occur on the outer perimeter of a silicon wafer as well as on its back side.
If not removed, such contamination can lead to cross-contamination of other wafers, via fabrication tools. Such contamination can be very difficult to remove, particularly if the contaminant has formed a stable silicide. It would be highly desirable if such contamination could be easily removed in a controlled manner without detrimentally affecting the front side of the workpiece.
The present invention provides processes for selectively treating surfaces of a workpiece having a first side, an opposing second side, and a peripheral edge defined between the perimeters of the first and second sides. In a first aspect of the present invention, a process is provided for applying a first fluid to the first side and peripheral edge of the workpiece, while excluding the first fluid from at least a majority of the second side of the workpiece. In a still further preferred embodiment, the first fluid is applied to the first side of the workpiece, the peripheral edge, and an outer perimeter portion of the second side of the workpiece. The first fluid preferably comprises an etchant to remove a metal film or oxide film from the exposed surface portions of the workpiece, to the exclusion of the remaining substantially non-exposed portion of the second side of the workpiece.
In a still further aspect of the present invention, a workpiece having a first side, an opposing second side, and a peripheral edge defined between the outer perimeters of the first side and the second side is received within a fluid chamber of a reactor. The fluid chamber has a first chamber portion which receives the first surface of the workpiece, and a second chamber portion which receives the second surface of the workpiece. A first fluid is supplied to the first chamber portion, in which the first side is exposed to the first fluid to the exclusion of the second side of the workpiece, which is not exposed in totality or to a predetermined extent to the first fluid. In the preferred embodiment, the first fluid includes an acid, preferably an inorganic acid, and an oxidizer that act on the first side to remove a metal film or oxide film therefrom, while not substantially affecting the second side of the workpiece or a selected portion of a second side of the workpiece. In addition to or in lieu of supplying the first fluid, a second fluid may optionally be supplied to the second chamber portion of the reactor, so that the second side of the workpiece or a selected portion of the second side of the workpiece is exposed to the second fluid. The second fluid may be a different process fluid such as an inert gas or liquid, a diluent or rinsing agent or other fluid.
The present invention thus provides a method and apparatus for selectively exposing a second side of a workpiece, such as a back side of a semiconductor wafer, to an etchant solution preferably including an etchant solvent, such as an acid, and optionally, an oxidizer, to remove a metal film, an oxide film or particulates from the back side of the wafer. The present invention also provides for exposure of the peripheral edge of the workpiece, such as the bevel edge of a semiconductor wafer, to the etchant solution to remove a metal film or oxide film from the bevel edge. Additionally, the processes and apparatus of the invention may be utilized to etch, remove, or reduce a metal film or an oxide film from a perimeter edge portion of the opposing second side of the workpiece, such as a narrow annular exclusion zone bordering the perimeter edge of the front (i.e., device) side of a semiconductor wafer. The selective exposures of surfaces of the workpiece are made without substantial exposure of the remainder of the second side of the workpiece, i.e., in the preferred embodiment, the device or front side of the semiconductor wafer. While the first fluid is supplied to the first side of the workpiece, the opposing second side of the workpiece may be exposed to no fluid, or may alternately be exposed to a purge fluid such as an inert gas or deonized water, or to another process fluid.
The present invention also provides semiconductor wafers and other workpieces produced from these processes.
The present invention also provides etchant solutions including an inorganic acid and ozone as an oxidizer, preferably hydrofluoric acid and ozone.
In a still further aspect of the invention, a processing fluid is selectively applied or excluded from an outer peripheral margin of at least one of the front or back sides of the workpiece. Exclusion and/or application of the processing fluid occurs by applying one or more processing fluids to the workpiece as the workpiece and corresponding reactor are spinning about an axis of rotation that is generally parallel (or antiparallel) to the vector defining the face of the workpiece being processed. The flow rate of the one or more processing fluids, fluid pressure, and/or spin rate are used to control the extent to which the processing fluid is selectively applied or excluded from the outer peripheral margin.
In a further aspect of the invention, a thin film is applied over the front side and over at least a portion of the outer perimeter. Usually, a barrier layer is applied over the front side and over at least a portion of the outer perimeter, whereupon a further thin film, such as a conductive seed layer, is applied over the barrier layer.
In a preferred embodiment, after one or more further intervening steps, such as electroplating of a metal layer onto the conductive seed layer, an etchant capable of removing one or more of the thin film layers is caused to flow over an outer margin of the front side while the etchant is prevented from flowing over the front side except for the outer margin. Thus, the etchant only contacts the outer margin of the front side thereby selectively removing only the one or more thin film layers from the outer margin of the front side. If the etchant is also caused to flow over the back side and over the outer perimeter, as well as over the outer margin of the front side, the one or more thin film layers are removed from the outer perimeter and any contaminant that the etchant is capable of removing is stripped from the back side as well.
Rather than an etchant, a cleaning chemical can be used in some applications to remove or dissolve the one or more thin film layers as described above.